In recent years, solid-state light sources and other narrow-wavelength-band and/or polarized light sources capable of producing visible light have drawn significant attention as alternative light sources to traditional light sources for use in imaging systems (such as projection systems). This attention has been due to many advantages of these light sources, such as compact size, greater durability, longer operating life, higher efficiency, and lower power consumption. For example, solid state sources such as LASERs, light-emitting-diodes (LEDs), and pumped non-linear optical crystals are increasingly being used or considered for use in imaging systems, e.g. imaging systems that employ one or more light valves each of which comprises an array of individually addressable pixels due to their low Etendue or low divergence. Solid state light sources enable illumination systems and display systems to have a reduced size and/or cost.
Regardless of certain superior properties over traditional light sources, solid-state light sources may produce unwanted artificial effects, one of which is speckle effect. Speckle patterns are often classified as either “objective” or “subjective” speckle depending on how they appear and behave. As coherent light propagates from a source to a detector plane it can encounter random path length differences that sum coherently to produce random intensity fluctuations that can be measured at the detector. These random intensity fluctuations often referred to as objective speckle, are independent of the viewer location. Subjective speckle is the random intensity variation that occurs within the viewer's vision due to the coherent light being viewed through the eye's aperture. Subjective speckle can be differentiated from objective speckle by the fact that the subjective speckle pattern changes as the viewer moves his head while viewing the speckle pattern.
In display applications using coherent light, such as light from solid-state illuminators, speckles accompanying the desired image displayed on a screen module the intensity of the desired image, and thus may significantly degrade the quality of the displayed image. Therefore, elimination or reduction of the speckle effect in display applications using phase-coherent light is highly desirable.